The curing technology involving-activation energy rays such as electron beams and ultraviolet rays has developed as an important technology in the fields of adhesives, coating agents, encapsulating materials, inks and sealing materials from the viewpoints of recent organic solvent emission control and reduction of consumption of production energy. Mainstream in these fields is a photocuring technology in which a monomer or an oligomer, such as a polyfunctional acrylate or an unsaturated polyester, is UV radically polymerized. Therefore, most studies focus on the UV radical polymerization. Resin compositions curable by the UV radical polymerization have characteristics of a high curing rate and a wide variety of applicable monomers and oligomers to permit preparation of cured products with various properties. However, the polymerization is easily inhibited by air oxygen, and the monomers and oligomers used are highly toxic and possess strong odor and skin irritation.
Accordingly, studies have been made on a UV cationic polymerization technology that is free of a problem of inhibited polymerization by air and can use less toxic and odorous monomers and oligomers. However, the UV cationic polymerization has a limited number of employable compounds and does not permit control of properties of cured products as the UV radical polymerization does. In particular, cured products by the UV cationic polymerization have the drawback of being inflexible.
To solve the above problems, means proposed for obtaining flexible cured products by the UV cationic polymerization include (1) an ultraviolet curing precision adhesive that contains specified amounts of an epoxy resin, an inorganic powder filler, a cationic photopolymerization initiator, and an intramolecularly epoxy-modified polybutadiene (JP-A-H03-252488); (2) a multiwire board adhesive that contains specified amounts of a specific epoxy resin, a polyfunctional epoxy resin with at least three epoxy groups, an intramolecularly epoxy-modified polybutadiene with at least three epoxy groups, a cationic photopolymerization initiator, and a tin compound (JP-A-H07-22751); and (3) a photocurable resin composition for sealing that essentially contains a resin whose main chain skeleton is composed of a butadiene homopolymer or a butadiene copolymer and which has an average of at least 1.5 epoxy groups in a molecular terminal and/or a side chain per molecule, an epoxy resin, and a photosensitive aromatic onium salt (JP-A-S60-217231).
The compositions of (1) to (3) have a common technical idea that a cured product is imparted with flexibility by internal plasticizing that is provided by the components compounded. The intramolecularly epoxy-modified polybutadiene, which is disclosed to be substantially useful in JP-A-H03-252488, is an epoxidized polybutadiene having an epoxy equivalent of 200 (corresponding to an epoxy number of 5 meq/g). The intramolecularly epoxy-modified polybutadiene with at least three epoxy groups, which is disclosed to be substantially useful in JP-A-H07-22751, is a polybutadiene having a molecular weight of 3000 and an epoxy equivalent of 200 (corresponding to an epoxy number of 5 meq/g). When the compositions containing such highly epoxidized polybutadienes are cured, the crosslink density of the epoxidized polybutadiene is increased to lower the elongation, and the cured products cannot display adequate flexibility. Moreover, the resin whose main chain skeleton is composed of a butadiene homopolymer or a butadiene copolymer and which has an average of at least 1.5 epoxy groups in a molecular terminal and/or a side chain per molecule, which is disclosed to be substantially useful in JP-A-S60-217231, is highly modified and hence the crosslink density is increased, so that a cured product cannot show adequate flexibility (Examples disclose an intramolecularly epoxidized polybutadiene having a molecular weight of 1500 and an epoxy oxirane oxygen content of 7.7% (corresponding to an epoxy number of 4.8 meq/g)). Further, the epoxy-terminated epoxidized polybutadiene, which is generally produced by reaction with epichlorohydrin, contains large amounts of impurities such as by-product chloride ions, so that the photocurable resin composition shows lowered humidity resistance and exhibits a corrosive action when used in contact with metal parts.
It is therefore an object of the present invention to provide a curable resin composition that shows high elongation and excellent rubber elasticity even in a cured state and has superior compatibility, transparency, flexibility and waterproofness.